1. Field of the Invention
The present invention relates to the gpD protein, which is the major subunit of the Duffy blood group antigen, and the use thereof in the detection and treatment of malaria.
2. Description of the Related Art
Malaria is the most prevalent infectious disease of mankind. Its widespread geographic distribution together with the severe pathologic consequences of the infection make malaria a major medical and financial burden for many of the developing nations.
There are several different kinds of malaria, one of which is caused by the parasite Plasmodium vivax, which attacks the red blood cells of susceptible individuals. A genetic trait of special interest with regard to P. vivax is the absence of antigens encoded by the blood group system called Duffy (F. B. Livingston, "The Duffy Blood Groups, Vivax Malaria and Malaria Sections in Human Populations: Review", Human Biol., 56, 413, (1984)). It has been shown that individuals whose red blood cells lack the product of the Duffy genes are not susceptible to the penetration of P. vivax owing to the fact that Duffy molecules serve as the receptor for the parasite. (L. H. Miller, H. J. Mason, D. F. Clyde and M. H. McGinnis, "The Resistance Factor to Plasmodium Vivax in Blacks, The Duffy Blood Group Genotype (a-b-)", N. Eng. J. Med., 295, 302, (1976)).
Malarial parasites are transmitted from host to host by blood sucking females of several species of the genus Anopheles. It is in the mosquito that the sexual phase of the life cycle of P. vivax takes place leading to the production of sporozoites. After their introduction into a "new" host, these sporozoites reside in the parenchymal cells of the liver and multiply asexually causing the eventual rupture of hepatic cells and the release of asexual forms (merozoites) into the blood stream. There the merozoites actively penetrate into red blood cells in a nearly synchronous fashion and because the rate of growth and cell division of P. vivax is essentially identical, the infected erythrocytes simultaneously reach the stage of parasite load at which they break. This produces the typical cycles of fever every 48 hours, hence the name of Tertian malaria.
P. vivax infection may persist without treatment for as long as five years. P. vivax parasitemias are relatively low-grade, primarily because the parasites favor the few young red blood cells or reticulocytes that exist in peripheral blood.
Immunity to P. vivax is commonly only partial in nature, which allows the occurrence of superinfections that evolve independently causing an overlap in the cycles of parasite release leading to the appearance of fever in shorter cycles. P. vivax exhibits considerable antigenic diversity and variation, as do other malarial Plasmodia (M. Hommel, Antigenic Variation in Malaria Parasites", Immunology Today, 6, 28, (1985)), although it has been recently shown that antigenic components of P. vivax sporozoites exist that are common to parasites from different isolates (F. Zavala, A. Masuda, P. M. Graves, V. Nussenzweig and R. Nussenzweig, "Ubiquity of the Repetitive Epitope of the CS Protein in Different Isolates of Human Malaria Parasites", J. Immunol., 135, 2790, (1985)).
In the context of the sources of antigenic differences between P. vivax isolates and their consequences with regard to vaccination, it is important that the merozoites of different strains of P. vivax share the same receptor for penetration into red blood cells, i.e., the Duffy molecule (Miller et al., N. Engl. J. Med., supra). In addition, regardless of its capacity to vary other antigenic molecules, the parasite recognition molecule, i.e., the molecule that binds to the Duffy molecule, must remain constant since it is the complementarity between it and the invariant receptor that allows the penetration of merozoites into erythrocytes and, thus, the continuity of the infection. Changes in the ligand specificity of this molecule would result in the loss of the parasite's capacity to infect, since P. vivax merozoites appear to be unable to utilize other human red blood cell receptors for their penetration in vivo, as shown by the resistance of Duffy negative erythrocytes.
The Duffy blood group system consists of two principal antigens Fy.sup.a and Fy.sup.b produced by Fy.sup.a and Fy.sup.b alleles. Antisera anti-Fy.sup.a and anti-Fy.sup.b defined four phenotypes, Fy(a+b-), Fy(a-b+), Fy(a+b+) and Fy(a-b-). W. L. Marsh, Crit. Rev. Clin. Lab. Sci., 5, 387 (1975). Neither antiserum agglutinates Duffy Fy(a-b-) cells, the predominate phenotype in blacks. Antisera defining the other phenotypes, Fy3, Fy4 and Fy5, are very rare. A murine monoclonal antibody, anti-Fy6, defined a new Duffy antigenic determinant present in all Duffy positive cells but absent in Fy(a-b-) cells. M. E. Nichols, P. Rubinstein, J. Barnwell, S. R. de Cordoba, and R. E. Rosenfield, J. Exp. Med., 166, 776 (1987). Blacks with Fy(a-b-) erythrocytes cannot be infected by P. vivax. These cells are also resistant to the in vitro invasion by P. knowlesi, a simian parasite that invades Fy(a+b-) and Fy(a-b+) human erythrocytes. L. H. Miller, S. J. Mason, J. A. Dvorak, M. H. McGinniss and K. I. Rothman, Science, 189, 561 (1985). Receptors for red cell invasion by these parasites, therefore, are related to the Duffy blood group system.